bpf: harden flags check in cgroup_storage_update_elem()
cgroup_storage_update_elem() shouldn't accept any flags
argument values except BPF_ANY and BPF_EXIST to guarantee
the backward compatibility, had a new flag value been added.
Fixes: de9cbbaadba5 ("bpf: introduce cgroup storage maps") Signed-off-by: Roman Gushchin <guro@fb.com> Reported-by: Daniel Borkmann <daniel@iogearbox.net> Cc: Alexei Starovoitov <ast@kernel.org> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>